| In In recent years,with the continuous construction of infrastructure,more and more projects need to be built on the soft soil foundation,and the requirements of soft soil reinforcement technology are higher and higher.Because of its convenient materials,low cost,and better drainage,replacement and other improvement functions,the reinforced gravel pile has been widely used in the treatment of soft foundation.However,the pile-soil interface of reinforced gravel pile involves the interaction of reinforcement,soil and stone.The calculation of pile side friction and the value of interface strength parameters are both empirical,and the related theoretical research is still far behind In engineering practice.Therefore,in this paper,based on the indoor large-scale direct shear test and the discrete element and discrete continuous coupling numerical simulation method,the strength characteristics and friction characteristics of the pile-soil interface of reinforced stirrup gravel pile are studied.The main research work is as follows:firstly,based on the large-scale direct shear apparatus in the laboratory,several groups of comparative tests are carried out to study theσand the w of soft soil around the pile,D_rof crushed stone and the setting of reinforcement on the pile-soil interface.The results show that:(1)theτof pile-soil interface increases linearly with the increase ofσ,but theμof pile-soil interface decreases with the increase ofσ,and tends to a certain limit value gradually.(2)Theτandμof the pile-soil interface increase with the decrease of the w of the soft soil around the pile.(3)Theτandμof pile-soil interface increase with the increase of relative compactness of crushed stone.The increase ofτandμof interface is mainly reflected in the increase of interface cohesion,which is mainly due to the occlusion and interlocking between dense particles.(4)The existence of reinforcement weakens the comprehensiveτandμof pile-soil interface,which is related to the contribution of the friction performance of reinforcement soil interface to the comprehensive friction performance of pile-soil interface.Secondly,considering that the discrete element method can reflect the stress and deformation mechanism of the specimen in the shear process from the microcosmic point of view,combined with fish language embedded in PFC,a numerical analysis model of crushed stone,geogrid and soft soil with the same mechanical properties as the indoor large-scale shear test is established.On the basis of verifying the rationality of the model,the particle geometry,relative compactness of crushed stone are analyzed The effects of micromechanical parameters of soft soil,particle to hole ratio and tensile rigidity of reinforcement on the friction characteristics of pile-soil interface are studied.The conclusions are as follows:(1)the parameter particle irregularity coefficient is defined,which can be used to describe the irregularity of gravel particle shape in reinforced concrete gravel pile quantitatively.It is found that when theσis small,theτof pile-soil interface increases linearly with the irregularity coefficient;and when theσis small,theτof pile-soil interface increases linearly with the irregularity coefficient The relationship between theτof pile-soil interface and the coefficient of irregularity shows a power function increase when the force is large,which shows that the larger theσis,the stronger the influence of the irregular shape of gravel particles on the interface performance is.Therefore,it is necessary to study the interface problem by considering the edge angle and the shape irregularity of particles.(2)The interface friction between soft soil and crushed stone is an important part of the comprehensive friction between pile and soil.Theτof soil stone interface and the comprehensiveτof pile-soil interface increase with the increase of the D_rof crushed stone,but the increase of theτof soil stone interface is more obvious.The friction angle of soil stone interface increases with the increase of D_r,but the cohesion of soil stone interface decreases with the increase of D_r.The change of cohesion is mainly due to the limited effect of reinforcement on particle displacement.In addition,it is found that the coordination number can not only verify the compactness of the numerical sample,but also reflect the micro deformation and stress characteristics of the particles in the shear process.(3)Theτof soft soil is closely related to the micro parameters of soil particles.The key factors are:particle contact modulus,bond strength,particle micro friction angle.Thirdly,using the discrete continuous coupling numerical simulation method,taking a soft foundation treatment project of an expressway as the prototype,the composite foundation analysis model composed of gravel pile(discrete)and soft soil around the pile(continuous)is established,and the magnitude and distribution rule of pile side friction are studied.The main conclusions are as follows:(1)vertical load transfer mechanism and pile of gravel pile and reinforced gravel pile The soil interaction is greatly affected by the radial swelling stress and deformation of the pile.(2)The distribution of radial bulging stress along the depth of gravel pile is very close to that of its radial bulging deformation,both of which reach the maximum value at2-3 times the pile diameter below the pile top,while the maximum radial stress and bulging deformation of reinforced gravel pile appear in a certain depth range of the end of reinforcement,and then gradually decline with the depth.(3)The radial bulging stress of pile-soil interface extracted from the soil is taken as theσof pile-soil interface.Combined with the interface strength parameters obtained from the study,the magnitude and distribution law of the side friction resistance of the reinforced hoop gravel pile and the common gravel pile with the depth are obtained.It is found that the pile side friction increases gradually along the depth,reaches the peak value and then decreases with the depth,which is similar to the change rule of the relative slip of pile-soil,indicating that this method of calculating the pile side friction can not only reflect the pile-soil relationshipFinally,based on theτparameters of the pile-soil interface of the stirrup and gravel piles obtained from the research in this paper,based on the calculation formulas of bearing capacity and settlement in the existing literature,the ribs before and after the modification of the interface strength parameters are calculated respectively The bearing capacity of the hoop crushed stone pile and the settlement value of the foundation were compared and analyzed with the field observation values.The study found that in the calculation of bearing capacity,insufficient consideration of lateral friction resistance is easy to underestimate the actual foundation bearing capacity,causing unnecessary waste;in the calculation of settlement,the settlement value obtained when the interface strength parameter is smaller is often more measured The value is too large.To sum up,the recommended values of the parameters of the pile-soil interface of the hoop crushed stone pile and the calculation method of the friction resistance of the pile side have positive significance for calculating the foundation bearing capacity and foundation settlement value more accurately. |
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